Conformational flexibility and molecular dynamics of cationic diolefin–rhodium(I) complexes with iminophosphine ligands

In solution, the cationic complexes [Rh(η2,η2-diolefin)(PN)]+ [diolefin=1,5-cyclooctadiene (cod), bicyclo[2.2.1]hepta-2,5-diene (nbd); PN=o-(PPh2)C6H4CHNR (R=C6H3(i-Pr)2-2,6 (1), CMe3 (2), (R)-bornyl (3)] undergo a conformational change of the iminophosphine ligand which inverts the position of the six-membered chelate ring relative to the NRhP coordination plane. The inversion is fast for [Rh(η2,η2-diolefin)(1)]+ and [Rh(η2,η2-diolefin)(3)]+ in the temperature range 298–183 K, and becomes progressively slower for [Rh(η2,η2-diolefin)(2)]+ with increasing steric demand of the coordinated ligands. From the coalescing signals in the 1H-NMR spectrum of [Rh(nbd)(2)]+ at 208 K, a ΔH# value of 28.5 kJ mol−1 and a ΔS# value of −60.3 J K−1 mol−1 can be estimated. The phase-sensitive 2D 1H-NMR ROESY spectrum of [Rh(cod)(3)]+ reveals the presence of another dynamic process which slowly and selectively interconverts the two olefinic protons on the same HCCH unit of the chelate diolefin. A mechanism is proposed involving the initial dissociation of the rhodium–olefin bond trans to phosphorus. Semi-empirical calculations show that in the most stable conformers of [Rh(nbd)(2)]+ and [Rh(cod)(2)]+ the PN chelate ring is not coplanar with the NRhP plane, while the diolefin coordination mode is distorted largely by the steric interaction with the bulky NCMe3 group. When the PN chelate ring is forced to be coplanar with the NRhP plane, the enthalpy content increases to 20.3 and 114.7 kJ mol−1 for [Rh(nbd)(2)]+ and [Rh(cod)(2)]+, respectively. Entropy factors seem to be predominant in the displacement of 1,5-cyclooctadiene by other olefinic ligands in the complexes [Rh(cod)(PN)]+.